Update to savon 2.3.0, remove namespace workaround

Savon fixed namespace parsing in https://github.com/savonrb/savon/pull/476
We should stop hardcoding the namespace in valvat in case the VIES service changes their WSDL

Also, turning off HTTPI logging individually is no longer needed (c.f. http://savonrb.com/version2/globals.html)